This invention relates to a catalyst and process for the polymerization of alpha-olefins, and more particularly, to a catalyst and process for the preparation of predominantly atactic polymers of alpha-olefins.
It is well known that atactic polymers of propylene and higher alpha-olefins are obtained as by-products in stereospecific polymerization operations and that the atactic and isotactic fractions of the total polymeric product can be separated due to different solubilities in hydrocarbon solvents. In typical slurry process stereospecific polymerization operations, the fraction of by-product atactic generally ranges up to about 10 wt.% of the total product and a number of uses for the atactic fraction have been developed; e.g., adhesives, waxes, artificial fireplace logs. However, in recent years, catalysts for the stereospecific polymerization of propylene and higher alpha-olefins have been improved to such an extent that the atactic fraction of the total product is so low that the same can be retained in the final product with only minor, if any, effects on the properties of the isotactic fraction. Given the greater commercial value of the isotactic fraction and the cost of separating isotactic and atactic fractions, there is little economic incentive to recover the atactic fraction in such operations. As commercial scale use of improved stereospecific catalysts increases, the supply of by-product atactic is likely to diminish and it may become difficult to fulfill the demand for atactic.
From the foregoing, it can be appreciated that a direct method for the preparation of atactic polyalpha-olefins would be desirable. It is an object of this invention to provide such a method. A further object of the invention is to provide a catalyst and process for the polymerization of alpha-olefins to predominantly atactic polymeric products. Other objects of the invention will be apparent from the following description.
We have now found that the foregoing objects can be attained according to the present invention wherein there is provided a catalyst for the polymerization of alpha-olefins to predominantly atactic products comprising (A) an organoaluminum component and (B) a second component which is the solid reaction product of components consisting essentially of (1) a titanium(IV) halide or alkoxyhalide and (2) a hydrocarbon-soluble organomagnesium-organoaluminum complex. According to a further aspect of the invention, there is provided a process for the preparation of predominantly atactic polymers of alpha-olefins comprising contacting at least one alpha-olefin of three or more carbon atoms with the above-described catalyst in the presence of an inert liquid polymerization medium. The polymeric products obtained in accordance with the present invention are predominantly atactic as evidenced by their solubility in alkanes. Typically the products are at least about 70 wt.% soluble in hexane. Further, the activity of the invented catalysts is sufficiently high that predominantly atactic polyalpha-olefins can be obtained at economically attractive production rates. Additionally, the molecular weights and molecular weight distributions of the predominantly atactic products obtained in accordance with this invention typically are at least comparable to those of by-product atactic produced in conventional slurry process isotactic polymerization operations, and accordingly the former are well suited for use in typical by-product atactic applications.
It is well known to polymerize alpha-olefins in the presence of catalytic mixtures of organoaluminum compounds with titanium(IV) compounds or with products obtained by reaction of titanium(IV) compounds with organometallic compounds; and organomagnesium-organoaluminum complexes of the type employed according to the present invention have been disclosed to be useful as "a cocatalyst for Ziegler-Natta systems." See Texas Alkyls Research and Development, Product Data Sheet "MAGALA-2B" and "MAGALA-6E," and U.S. Pat. No. 3,737,393 (de Vries), U.S. Pat. No. 4,128,501 (Smith et al.), and Canadian Pat. No. 977,766 (Texas Alkyls). However, such catalysts typically are useful in the production of polyethylene or predominantly isotactic polymers of propylene and higher alpha-olefins. For example, Example 8 of the aforesaid Canadian Pat. No. 977,766 illustrates polymerization of propylene using a catalytic mixture of titanium tetrachloride, an approximately 1.7:1 complex of di-n-butylmagnesium and triethylaluminum, and ethylaluminum sesquichloride to a product having an isotactic index of 93 percent. Accordingly, it was unexpected that predominantly atactic products would be obtained through the use of the catalysts of this invention.
Other prior art which may be of interest to the present invention in disclosing the use of organomagnesium-organoaluminum complexes in preparation of alpha-olefin polymerization catalyst components include the following.
British Pat. No. 1,492,379 (Dow) discloses alpha-olefin polymerization catalysts comprising (A) the reaction product of (1) an ester of titanium(III) or (IV) with (2) an intermediate reaction product of (a) an organomagnesium compound, including hydrocarbon-soluble complexes of dialkylmagnesiums with trialkylaluminums and (b) a metallic halide such as a dialkylaluminum halide or alkylaluminum dihalide; or (B) the reaction product of (1) the intermediate reaction product of (a) the titanium ester with (b) the organomagnesium component, and (2) the metallic halide. In the polymerization of alpha-olefins, the disclosed catalysts can be employed alone or "in combination with a common Ziegler catalyst activator such as an organoaluminum compound." According to the patentee, the disclosed catalysts are useful primarily in the polymerization of ethylene.
British Pat. No. 1,500,873 (Dow) discloses alpha-olefin polymerization catalysts comprising (1) an aluminum alkyl component and (2) a component which is the reaction product of (a) a transition metal compound, including titanium halides and alkoxides, (b) a hydrocarbon-soluble organomagnesium compound or complex thereof with another organometallic such as a trialkylaluminum, and (c) a hydrogen halide or organic halide. According to the patentee, the disclosed catalysts give best results in the polymerization of ethylene.
U.S. Pat. No. 4,135,046 (Harris et al.) discloses an ethylene polymerization catalyst prepared by reducing tetrabutyl titanate with a 1:2 complex of a dialkylmagnesium and a trialkylaluminum followed by deactivating excess dialkylmagnesium with hydrogen chloride and then adding a second increment of tetrabutyl titanate. For use in the polymerization of ethylene, the resulting catalyst is employed without addition of an aluminum alkyl component.
Although the foregoing patents disclose the use of titanium(IV) compounds and organomagnesium-organoaluminum complexes in preparation of alpha-olefin polymerization catalyst components, each of the patents also requires the use of an alkylaluminum halide or a nonmetallic halide in preparation. In contrast, the solid component of the catalysts of the present invention is prepared without the use of such materials. Further, the catalysts disclosed in the aforesaid patents are primarily useful in the polymerization of ethylene. As a general rule, typical ethylene polymerization catalysts are of little use in operations for either stereospecific or atactic polymerization of propylene and higher alpha-olefins due to relatively low activities and because the atactic and isotactic fractions resulting from such operations often are relatively close.
The following patents, although not directed to the use of organomagnesium-organoaluminum complexes in preparation of alpha-olefin polymerization catalyst components and/or catalysts, may be of interest to the present invention in disclosing the use of magnesium and/or various compounds thereof in preparation of catalyst components, and/or catalysts for the production of atactic polyalpha-olefins.
U.S. Pat. No. 3,933,934 (Bailly et al.) discloses catalysts for the production of atactic polyalpha-olefin waxes comprising (1) a Group II or III organometallic such as an aluminum alkyl and (2) the solid product obtained by reacting (a) a titanium halide, alkoxide, or alkoxyhalide, (b) metallic magnesium, and (c) an alkyl halide. Polymerization of alpha-olefins in the presence of the disclosed catalysts is reported to result in high yields of predominantly atactic polyalpha-olefins as evidenced by their solubility in alkanes (at least about 95% soluble in boiling heptane). However, such catalysts are disadvantageous from the standpoint of preparative ease and economy because separation of the solid catalyst component from unreacted magnesium and removal of reaction by-products can be difficult.
U.S. Pat. No. 3,951,935 (Engelmann) discloses catalysts for the preparation of atactic polyalpha-olefins comprising (1) a hydrocarbylaluminum halide and (2) a component prepared from (a) titanium tetrachloride or a titanium tetraalkoxide, (b) a magnesium compound containing hydroxy or alkoxy groups, and optionally, (c) a silicon and/or aluminum chloride or alkoxide. Polymerization in the presence of the disclosed catalysts is conducted at 100.degree. to 160.degree. C., and in Example 1 it is reported that an atactic polypropylene fraction of 75 wt.% was obtained.
U.S. Pat. No. 3,101,328 (Edmonds, Jr.) discloses polymerization of alpha-olefins in the presence of a catalytic mixture of an organic halide, a Group IV metal halide such as titanium trichloride or titanium tetrachloride, and a Group I-III metal (e.g., magnesium) or mixture thereof. Polymer tacticity is not discussed.
U.S. Pat. No. 3,179,601 (Kummer) discloses a brown solid ethylene polymerization catalyst prepared by reaction of titanium tetrahalide vapors with magnesium vapors. Again, product tacticity is not discussed.
U.S. Pat. No. 3,801,558 (Fletcher et al.) discloses alpha-olefin polymerization catalysts comprising an organoaluminum component and a component prepared by reaction of titanium tetrachloride with a hydrocarbon soluble organomagnesium compound prepared by reacting metallic magnesium with an alkyl halide in the presence of a hydrocarbon solvent and a complexing agent. Again, product tacticity is not discussed.